Regenerable Capacitor

ABSTRACT

A regenerable electric capacitor having wound-on layers of plastic foils provided with metal layers of the coatings. The metal layers consist of an alloy and include a variable thickness perpendicular to the longitudinal direction of the foils. As such, the thickness of the metal layers is smallest in the regions bordering metal-free edge strips and it increases towards the opposite side of the foil. In addition, the metal layers have an alloy composition that is different dependent transverse to the direction of run of the foils.

This application is a 371 of PCT/DE97/01665 filed Aug. 7, 1997.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a regenerable electric capacitor thathas metal layers that have thicknesses and alloy compositions which aredifferent dependent transverse to a direction of run of wound-on layersof plastic foils.

2. Description of the Prior Art

The possibilities of use of power capacitors are determined essentiallyby their thermal characteristics. In self-healing, or regenerable,capacitors, the greatest part of the losses arises in the metal layersof the coatings.

In order to minimize the losses, the metal layers can be made thickerand the surface resistances made smaller. However, for reasons ofregeneration reliability, narrow limits are placed on this proceduresince, beginning at a certain thickness of the metal layers a correctregenerability is no longer ensured. It is thus generally known that thethickness of at least one metal layer must be kept thin.

Improvements have resulted by means of a capacitor of the type namedabove known from DE 28 26 481 C2. in the capacitor depicted there, themetal layers, consisting of an alloy of 15-80 atomic % aluminum withcopper, include a variable thickness perpendicular to the longitudinaldirection of the foils) i.e., transverse to the direction of run of thecapacitor foil) so that the cross-sectional profile of the metal layersis essentially wedge-shaped with a constantly increasing thickness.However, with respect to the regeneration characteristic, the mostunfavorable conditions hereby prevail in the center so that the minimalallowable surface resistance is determined at this point.

In addition, from EP 0 088 137 A1, a self-healing electric capacitor isknown in which the metal layers consist of an Al/Zn alloy whereby the Alportion constantly decreases from 80% at the side of the metal layerfacing the foil to less than 20% at the side of the metal layer facingaway from the foil.

If the manufacturing of wound capacitors is regarded from an economicalpoint of view, it is to be noted that the manufacture of larger windingunits is generally more cost-effective. For this reason, it is desirableto manufacture the largest possible winding units with largeoverlappings of the oppositely poled metal layers. However, for thethermal economy of the capacitor there results the disadvantageouseffect that the series losses increase proportionally to the size of thecovering so that an optimization problem between the economical windingmanufacture and the thermal requirements occurs.

An object of the present invention is, therefore, to develop a capacitorthat, in comparison with the prior art, exhibits an at least equallygood, but if possible a better, regeneration characteristic and anequally good, or better, life span. The operation of such capacitorshould be such that minimal coating losses arise which thus enable ahigher exploitation of the dielectric material as well as costreductions.

SUMMARY OF THE INVENTION

Accordingly, in an embodiment of the present invention, a regenerableelectric capacitor is provided which includes: wound-on layers ofplastic foils; metal-free edge strips respectively arranged onlongitudinal sides of the foils; and metal layers of the coatingsrespectively provided on the foils, wherein the metal layers are formedof an alloy and include a variable thickness perpendicular to alongitudinal direction of the foils, the thickness of the metal layersbeing smallest in regions bordering the metal-free edge strips andincreasing towards an opposite foil side, the foils being would with oneanother such that given two foils lying on one another the metal-freeedge strips are arranged on different frontal sides of the capacitor,the metal layers having a thickness and alloy composition that isdifferent dependent transverse to a direction of run of the foils suchthat the thickness and the alloy composition vary from the regionconnected to the metal-free edge strips to the opposite foil side, andthe metal layers are profiled in stepped form and are formed of azinc/aluminum alloy in that the aluminum portion of the alloy increasesfrom a value lower than 5% to a value higher than 10% (weightpercentage).

In an embodiment, the metal layers additionally contain silver.

In an embodiment, the metal layers additionally contain copper.

In an embodiment, the metal layers additionally contain magnesium.

In an embodiment, the silver portion is contained as at least oneuniformly embedded layer.

In an embodiment, the silver portion is placed in as a doping.

In an embodiment, the aluminum portion is smallest in the regionsbordering the metal-free edge strips.

In an embodiment, a respective half increase of the portion of themetals of the alloy composition is achieved at a location outside acenter of the foil.

In an embodiment, the metal layers form a structuring.

In an embodiment, the plastic foils are provided with a wave cut in theregion connected to the metal-free edge strips.

Additional features and advantages of the present invention aredescribed in, and will be apparent from, the Detailed Description of thePreferred Embodiments and the Drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the schematic cross section through two foil layers of thecapacitor of the present invention.

FIG. 2 shows the qualitative distribution of the alloy components forthe capacitor foil.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a cross section through two plastic foils 1, 2 lying on oneanother, which are respectively provided with metal layers 3, 4. Thethickness of the metal layers 3, 4 is shown in distorted fashion incomparison to the thickness of the foils 1, 2. In reality, the metallayers 3, 4 are significantly thinner than the plastic foils 1, 2. Thethickness of the metal layers 3, 4 is smallest in the region connectedto metal-free edge strips 5, 6 and increases in stepped form towards theopposite side of the foil. At this point, the metal layers 3, 4 arethickest and are contacted with schoop layers not are shown in FIG. 1.

However, it is also possible to apply the metal layers 3, 4 in such away that after the first step 7, 8, their thickness becomes that whichis shown in dotted lines. The region in which the first step 7, 8 beginsis displaced by an amount z in relation to the geometrical center (halfwinding width WB/2 of the foils 1, 2). In order to prevent thepossibility of short-circuits between the metal layers 3, 4, the foils1, 2 are wound on with a certain offset V to one another.

The capacitatively effective region of the capacitor is determined bythe degree of overlap Ü of the metal layers 3, 4, which results from thewidth x of the metal layers 3, 4 minus the free edge region FR and theoffset V. The surface resistance is greatest in the thin region of themetal layers 3, 4 (R_(max)) and is lowest in the thickest region(R_(min)).

FIG. 2 shows the quantitative distribution, (weight percentage), of thealloy components aluminum (Al) and zinc (Zn). The free edge region islocated at the right in FIG. 2 wherein the zinc portion is greater thanthe aluminum portion. The composition of the alloy components is therebyselected in such a way that the higher aluminum portion is achievedapproximately in the center of the foils.

In addition to the alloy components Al/Zn, the metal layers also cancontain silver. The silver is present either as uniformly embeddedlayer(s)/blocking layer/ or as a doping portion distributed eitheruniformly or non-uniformly in the Al/Zn alloy.

Concerning the calculation of the cross-profiling of the metal layers asa function of x, the coating losses can be calculated with the followingsolution approach:

the displacement current I₀ that flows into the coating from thecontacting per unit of length decreases proportional to x from I₀=max atx=0 to I₀=0 at x=(Ü+FR+V). In a surface element dI·xdx with the surfaceresistance R, the power loss P is consumed. As such, the followingholds: $\begin{matrix}(1) & {{{P} = {I^{2}{R}}},} \\(2) & {{I_{x} = {I\quad \frac{x}{\overset{..}{u}}}},} \\(3) & {{R} = {\frac{\rho}{A}\quad {x}}}\end{matrix}$

From 1 to 3 there results: $\begin{matrix}\text{(1-3)} & {{P} = {\left( {I_{0}\quad \frac{x}{\overset{..}{u}}} \right)^{2}\frac{\rho}{A}\quad {x}}} \\(4) & {{P = {{I_{0}^{2}\quad \frac{\rho}{A\overset{..}{u}}\quad {\int_{0}^{\overset{..}{u}}{x^{2}\quad {x}}}} = {\frac{1}{3}\quad \frac{\overset{..}{u}}{G_{0}I}\quad {I_{0}^{2}.}}}}\quad}\end{matrix}$

For the metal layers 3, 4 with the surface resistances R_(3,4) thereresults the power loss:

P _(hB)≈⅔I ₀ ²·1.00·R _(hB)  (5)

(with R_(hB)=const=>homogenous coating)

Pk _(B)≈⅔I ₀ ²·0.75·R _(kB)  (6)

(with R_(kB)=R₀+R₀·x≈wedge-shaped coating)

P _(sB)≈⅔I ₀ ²·0.40·R _(sB)  (7)

(with R_(sB), the coating profiled in stepped form according to FIG. 1,and an alloy composition as a function of x according to FIG. 2).

Equation (7) shows a significantly reduced power loss production incomparison to the known metallization profiles. Alongside an optimallyconstructed metallization profiling according to FIG. 1, this advantageis also effected by the modification of the main alloy componentsaluminum and zinc (if necessary, for example, with portions of silver)dependent on x according to FIG. 2.

The alloy composition as a function of x contributes to the solution ofthe underlying object of the present invention because, in particularfor thin layers, the layer-thickness-related surface resistance of zincand aluminum differs by approximately a factor of 2, i.e.; R_(Al)<R_(Zn)with d=const. In addition, given the alloy metallization Zn/Al, thereexists the effect that for small Al portions (approximately <5% Al) thisaluminum portion acts as an imperfection in the Zn grid structure, andR_(Zn/Al) is increased superproportionally to the existing layerthickness. If the aluminum portion in the thicker region of the steppedmetallization is now increased to a maximum allowable value(approximately >10% Al), taking into account the requirement ΔC/C(t,T,E,. . . )=const, then the surface resistance is consequentially reduced ina manner superproportional to the existing Zn layer thickness, sincebeginning from a threshold value, the Al embedding in the Zn grid has asignificant effect on a reduction of the surface resistance R.

The allocation of the alloy composition in the x direction also can beoptimally organized according to the desired application (AC, DC, SKcapacitors) from the point of view of manufacturing the metal layers.From the qualitative curve of the alloy components shown in FIG. 2, itcan be seen that the distance z in relation to the midaxis is taken intoaccount as a security overlapping of the thin coating equipped with goodregeneration characteristics.

As already mentioned, the alloy metallizations also can include silverportions which, analogous to the Zn/Al alloy, can represent a functionof x. Mainly in the region of the thin metallization (region R_(max)),advantages emerge if silver is used in increased measure since thelayer-thickness-related surface resistance is reducedsuperproportionally and effectively. In the transition zone from thickto thin metallizations, i.e. in the region of the degree of overlap ü,silver is particularly advantageous.

The optimization of the cross-profiling also takes into account thepower loss production as a function of x. The calculation of the formula(7) with different surface resistance ratios R_(max)/R_(min) shows that,in particular at high R_(max)/R_(min) ratios, load peaks occur in theregion WB/2. On the basis of these relationships, the carefulcalculation or, respectively, optimization of all parameters is requiredin order to make effective use of the described advantages in thetechnical product; such as, for example, capacitor windings for ACapplications with a winding width of approximately 50 mm to 170 mm.

The advantages described also result in a further improvement of, forexample, capacitors with structured coatings and/or capacitor foils witha wave cut in the free edge area.

In addition, it is also possible to manufacture the described structureof the metal layers in a suitable manner as a metallization on bothsides with an offset free edge. As such, the thick layer is arranged inthe contact zone.

On the other hand, it is also possible to combine a very thinmetallization (for example 15-30Ω) with a free edge with the structuredepicted in the embodiment according to FIG. 1 on the second side of thefoil.

Although other present invention has been described with reference tospecific embodiments, those of skill in the art will recognize thatchanges may be made thereto without departing from the spirit and scopeof the invention as set forth in the hereafter appended claims.

What is claimed is:
 1. The regenerable electric capacitor, comprising:wound-on layers of plastic foils; metal-free edge strips respectivelyarranged on longitudinal sides of the foils; and metal layersrespectively provided on the foils, wherein the metal layers are formedof an alloy having a variable thickness perpendicular to a longitudinaldirection of the foils, the thickness of the metal layers being small inregions bordering the metal-free edge strips and increasing towards anopposite side of the foils where each foil has a large thickness, thefoils being wound with one another such that given two foils lying onone another the metal-free edge strips are arranged on different frontalsides of the capacitor, the metal layers having a thickness and alloycomposition that changes transverse to said longitudinal direction ofthe foils such that the thickness and the alloy composition vary fromthe region connected to the metal-free edge strips to the opposite sideof each foil, and the metal layers are profiled in stepped form and areformed of a zinc/aluminum alloy such that a weight percentage of thealuminum portion of the alloy increases from a value less than 5% at thesmall side of each metal layer to a value greater than 10% at the largethickness side of each metal layer.
 2. The regenerable electriccapacitor as claimed in claim 1 wherein the metal layers additionallycontain silver.
 3. The regenerable electric capacitor as claimed inclaim 2 wherein said metal layers are doped with the silver.
 4. Theregenerable electric capacitor as claimed in claim 1 wherein the metallayers additionally contain copper.
 5. The regenerable electriccapacitor as claimed in claim 1 wherein the metal layers additionallycontain magnesium.
 6. The regenerable electric capacitor as claimed inclaim 2 wherein the silver is contain as at least one uniformly embeddedlayer.
 7. The regenerable electric capacitor as claimed in claim 1wherein the aluminum portion is smallest in the regions bordering themetal-free edge strips.
 8. The regenerable electric capacitor as claimedin claim 1 wherein half of the increase of the aluminum weightpercentage of the aluminum portion is achieved at a location adjacent toa center of each foil.
 9. The regenerable electric capacitor as claimedin claim 1 wherein the metal layers exhibit a surface structure.